The present invention relates to method of multi-color recording using an electro-photography process and apparatuses therefor and, more particularly to a method of multi-color recording using an electro-photography process and an apparatus therefor in which, the second and successive charging processes after a second color image forming process are performed by using a recharging means with a controlling electrode.
In a prior art multi-color recording apparatus using an electro-photography process, in general the toner images having multiple colors are formed by overlapping the multiple colors on a surface of an endless type photosensitive body and the multi-color toners formed on the surface of the photosensitive body are transferred to a recording medium such as recording paper.
In the above stated prior art multi-color recording apparatus using the electro-photography process, the multiple image producing units are installed along a peripheral portion of a rotatable and cylindrically shaped photosensitive body.
One image producing unit of the multiple image producing units is formed with a set structure comprising a charging means, an exposing means and a developing means, for example.
Furthermore, a transferring means and a fixing means are arranged respectively at a rear position of the installing position of the final stage of the multiple image producing units.
In the above stated prior art multi-color recording apparatus using the electro-photography process, by using the mulitple image producing units, the toner images having the mulitple colors are formed on the surface of the photosensitive body. The toner images having the multiple colors are then transferred to the recording paper at a transferring portion. Next, by using the fixing means the toners are fused thermally and adhered to the recording paper. In this way, the multi-color recording printing method using the electro-photography process is carried out.
In addition, as a first treatment for the image production using the image producing units, the charging means for use in the charging process is constituted so as to charge uniformly the electric charges as much as possible on the surface of the photosensitive body.
Besides, the exposing means exposes selectively the electric charges on the surface of the photosensitive body in accordance with the recording contents (character, figure etc.). In the exposing means, by selectively erasing the electric charges on the surface of the photosensitive body, electro-static latent images are formed on the surface of the photosensitive body.
In addition, the developing means adheres the toners with the electro-static latent images which have been formed on the surface of the photosensitive body. Accordingly, the developing means forms the toner images on the surface of the photosensitive body.
Herein, it will be explained about the charging in the image producing process on and after the second color and on and after the second time. At a portion of the surface of the photosensitive body in which the toner images are formed by using the image producing unit of the prior stage, the toner image portion has a low surface potential (absolute value) in comparison with the surface potential of other areas (non-image portions) because in the toner image portion the electric charges are erased by the exposing process.
In the present invention, the toner image portion refers to the portion of the surface of the photosensitive body in which the toner images are formed, and the non-image portion refers to the portion of the surface of the photosensitive body to which the toners is not adhered and also a portion on which the toner images are not formed, respectively.
Accordingly, in the charging means of the image producing unit on and after second color (hereinafter, it is referred to as re-charging means in the present invention), it is desirable to uniformly charge the surface of the photosensitive body as much as possible, despite that the surface potential of the photosensitive body is not always uniform. Furthermore, it is desirable to increase the surface potential of the non-image portion to a predetermined target surface potential Vo.
In the prior art re-charging method, a scorotron type charging means with a controlling electrode (grid electrode) is employed as the re-charging means.
In this scorotron type charging means, the grid potential Vg is set substantially equal to the predetermined target surface potential Vo and in comparison with the non-image portion many corona discharging currents are made to flow into the exposing portion (the toner image portion) having the low surface potential, thereby making the surface potential of the photosensitive body uniform uniformly.
However, when a high speed (for example, 150.about.2000 mm/s) printing speed is attempted in the multi-color recording apparatus using the electro-photography process according to the prior art re-charging method, in the developing process of each image producing unit from the second color and thereafter, the problem occurs that the toners of the this time stage (the present stage) mix with the toner images that have been formed on the surface of the photosensitive body from the earlier stage.
Accordingly it is difficult to perform fine multi-color printing in the multi-color recording apparatus using the electro-photography process due to the mixed colors. In particular to ensure the proper toner image density of the present time stage, when the developing bias potential is high, it has a tendency to increase the mixed colors.
With respect to the cause of the mixed colors, the inventors of the present invention performed various experiments. As a result, the inventors have found the following points.
First, when operating at the high speed, it become the short time for charging by the re-charging means is short. Accordingly, before the electric charges are supplied sufficiently to the toner image portion of the surface of the photosensitive body as shown in FIG. 4B, the re-charging process has been completed using the prior art re-charging means.
Accordingly, the surface potential Vi of the toner image portion of the prior before stage (the surface potential Vb after the re-charging) does not rise sufficiently and is left at the condition of a large surface potential difference .DELTA.Vb=.vertline.Vo-Vb.vertline. (difference between the surface potential Vo of the non-image portion (such as a background portion) after the re-charging and the surface potential Vb of the toner image portion of the after later stage).
If the large surface potential difference .DELTA.Vb is left as it is, the exposing process and the toner developing process of the current time stage is carried out.
As a result, the surface potential Vb of the toner image portion after the re-charging is lower than the direct current component potential Vdc of the developing bias potential. Accordingly the toners of the current time stage are mixed into the toner image portion of the prior before stage and causes the mixed colors.
As a method for preventing the mixed colors in combination with the above stated high speed printing in the multi-color recording apparatus using the electro-photography process, in the first place, by using a large size re-charging means it is possible to maintain sufficiently the charging time, however, this solution requires a large size multi-color recording apparatus.
The the technique about an improvement of the multi-color recording apparatus having a scorotron type charger as the charging means is shown in, for example Japanese patent laid-open No. 116,793/1984.
In the above Japanese patent laid-open publication, a charging process in the multi-color recording apparatus is proposed that applies the potential to the grid wires. This such applied potential has substantially the same potential value of the target charging potential and, the re-charging is carried out at the corona discharging area accompanying the light generation.
However, in the above stated charging process, in case of a negative corona discharge or in a case in which the corona wire or the grid wire is adhered by a dirty substance, then the light generation distribution becomes non-uniform and the potential after the re-charging has a tendency to become unstable.
Second, if the current capacity is increased in the scorotron type charging means used as the re-charging means, then the corona discharging current Ic is increased and the surface potential Vi of the toner image portion formed at the prior stage is increased sufficiently to increase the surface potential Vb of the toner image portion after the re-charging.
However, in a case in which it is desired to have the large electric capacity of the re-charging means and to increase the corona discharging electric Ic in the re-charging means, then the surface potential Vn of the non-image portion after the re-charging is made excessively high.
Accordingly, that it decreases the insulating property of the photosensitive body and the life of the photosensitive body is shortened. It also causes the black points on the photosensitive body or on the recording paper and lowers the developing property of small characters.
Also, in the above stated prior art multi-color recording apparatus using the electro-photography process, when the charging conditions are changed in concert with the change in the environmental conditions, the change in the surface conditions of the photosensitive body and the change in the dirty states of the re-charging means etc. due to the scattering of the toners, the quality of the multi-color printing material has a tendency to fluctuate.